Topographic Control of Groundwater Flow
Abstract: Gravity is the main driving force for groundwater flow, and both landscape topography and geology distribute the effects of gravity on groundwater flow. The groundwater table defines the distribution of the potential energy of the water. In humid regions where the bedrock permeability is relatively low and the soil depth is sufficiently shallow, the groundwater table closely follows the landscape topography and, thus, the topography controls the groundwater circulation in these regions. In this thesis, I investigate multi-scale topography-controlled groundwater flow, with the goal of systematizing the spatial distribution of groundwater flow and assessing geological parameters of importance for groundwater circulation. Both exact solutions and numerical models are utilized for analyzing topography-controlled groundwater flow. The more complex numerical models are used to explore the importance of various simplifications of the exact solutions. The exact solutions are based on spectral representation of the topography and superpositioning of unit solutions to the groundwater flow field. This approach is an efficient way to analyze multi-scaled topography-controlled groundwater flow because the impact of individual topographic scales on the groundwater flow can be analyzed separately. The results presented here indicate that topography is fractal and affects groundwater flow cells at wide range of spatial scales. We show that the fractal nature of the land surface produces fractal distributions of the subsurface flow patterns. This underlying similarity in hydrological processes also yields a single scale-independent distribution of subsurface water residence times which have been found in distributions of solute efflux from watersheds. Geological trends modify the topographic control of the groundwater circulation pattern and this thesis presents exact solutions explaining the impact of geological layering, depth-decaying and anisotropic hydraulic conductivity on the groundwater flow field. For instance, layers of Quaternary deposits and decaying permeability with depth both increase the importance of smaller topographic scales and creates groundwater flow fields where a larger portion of the water occupies smaller and shallower circulation cells, in comparison to homogeneous systems.
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